When designing a midsole of a shoe, various considerations must be taken into account. Unfortunately, some of the considerations work to the disadvantage of others. For example, depending on the application, a shoe can be designed to dissipate a predetermined amount of shock by choosing the appropriate hardness of the material within the midsole. However, the midsole should not be so soft that it sacrifices lateral stability. When manufacturing midsoles according to the methods known in the prior art, sufficient stability and shock absorption can usually be achieved through a compromise; however, the ornamental cosmetic features on the exterior of the midsole are consequently sacrificed. This is due to the way that midsoles are conventionally manufactured.
In particular, prior methods of manufacturing midsoles first require the production of a sheet stock of plastic material such as an open cell foam or a closed cell foam. These plastic sheet stock materials generally consist of well known open cell foam materials such as crepe rubber or polyurethane (PU). Alternatively, well known closed cell foam materials including ethylene vinyl acetate (EVA) could be used. These open and closed cell foam materials are discussed in more detail in U.S. Pat. No. 4,043,058 issued to Hollister et al. and U.S. Pat. No. 4,128,950 issued to Bowerman et al., both of which are hereby incorporated by reference.
Hollister et al. '058 and Bowerman et al. '950 disclose a midsole construction whose component parts are stockfit. In other words, the component pieces comprising the midsole are molded from a closed cell foam material such as EVA or polyisoprene microcellular rubber, die out from the closed cell foam materials, and cemented together to form a unitary midsole. Giese et al., U.S. Pat. No. 4,316,332 and Adamik et al., U.S. Pat. No. 4,302,892 also disclose midsoles whose components are cemented together in a stockfitting process. Giese et al., U.S. Pat. No. 4,316,332 and Adamik et al., U.S. Pat. No. 4,302,892 are hereby incorporated by reference. Also incorporated by reference are U.S. Pat. No. 3,821,135 to King, U.S. Pat. No. 4,364,189 to Bates and U.S. Pat. No. 4,228,600 to Krug et al. which disclose midsoles manufactured from more than one material.
Once a sheet stock of the plastic material is manufactured, it is cut into a plurality of pieces which approximate the shape of a midsole. These midsole pieces are known as preforms. The preforms are placed within a mold half such as that shown in FIG. 2. The mold half 10 has a cut-out portion 12 of the same shape of the completed midsole. Another unshown mold half applies vertically directed pressure with respect to the top surface 14 of the mold half 10. The arrow 16 shows the general direction of pressure applied to the preform as it sits in cut-out portion 12. Consequently, the preform is compressed within the mold half 10. In this compression molding process, the preform may have a compression set within the range of about 12% to 15%. Alternatively, the preform may be, for example, 105% of the final product formed by the compression molding.
The compression molding relies primarily on heat and pressure to reduce the size of the original cell structure of the fabricated component by minimizing the amount of air and/or gas within the individual cells. The compression, which is similar to preshriking or sanforizing, maintains the life of the material such as EVA over a period of time longer than non-compressed EVA preforms. This molding process is described in more detail in U.S. Pat. Nos. 4,730,402 and 4,876,053, both of which are issued to Norton et al. and both of which are hereby incorporated by reference. Furthermore, the process of molding EVA with polyurethane is also disclosed in U.S. Pat. Nos. 4,551,930, 4,561,140, and 4,654,983, all issued to Graham et al. and all of which are hereby incorporated by reference.
While there are advantages gained by compression molding a plastic material, e.g. extending the life of the material, there are attendant disadvantages of such compressive forces. In particular, the compressive forces are aligned in the direction of arrow 16 which is substantially parallel to the inner surface 18 of the sidewall of the mold 10. Consequently, the outer surface of the perform receives very little lateral compressive forces. Instead, the outer surface of the preform aligned along the inner surface 18 of the mold 10 receives mostly vertically directed compressive forces.
Consequently, it is very difficult to obtain ornamental details on the sidewalls of the midsole which have a high resolution. Instead, only nominal details can be formed on the side of a preform, because the compressive forces imparted on the preform are not aligned in the proper direction. Therefore, in order to develop fine details on the outer surface of the midsole, it is necessary to apply the compressive forces in a direction which is aligned substantially perpendicular with respect to the outer surface of the midsole.